Many eye diseases are currently treated with lasers or other similar visible light sources. The most common treatments involve thermal destruction of areas of diseased cells within the eye. When the diseased cells are destroyed local progression of the disease is stopped and in some cases, cellular signaling induces a healing response and healthy cells replace the diseased ones. A drawback to these common thermal therapeutic techniques is due to the proximity of the diseased cells to the healthy cells. During the course of the therapeutic treatment, many healthy cells are unintentionally destroyed due to the imprecise thermal effect of the laser.
In an attempt to reduce unintentional thermal damage to proximal cells, a new therapeutic technique is currently gaining prominence. The new technique is called Photodynamic Therapy (PDT). Photodynamic therapy or PDT is a process whereby light of a specific wavelength or waveband is directed to tissues undergoing treatment or investigation that have been rendered photosensitive through the administration of a photoreactive or photosensitizing agent. In this therapy, a photoreactive agent having a characteristic light absorption waveband is first administered to the patient, typically either orally or by injection or even by local delivery to the treatment site. Proliferating cells, such as those involved in many eye diseases, may preferentially take up or absorb a number of or many photosensitizing agents. Once the drug or photosensitizing agent has been administered and reaches the targeted tissue, the tissue is illuminated with light of an appropriate wavelength or waveband corresponding to the absorption wavelength or waveband of the photoreactive agent.
The objective of the PDT may be either diagnostic, where the energy level and wavelength of light is selected to cause the photoreactive agent to fluoresce, thus yielding information about the tissue without damaging the tissue, or therapeutic, where the wavelength of light delivered to the photosensitive tissue under treatment causes the photoreactive agent to undergo a photochemical interaction with oxygen in the tissue under treatment yielding free radical species such as singlet oxygen, causing local tissue effect.
In PDT therapies, the photosensitizing agent or drug is injected into the body of the patient, and then light at lower than thermal energy levels is focused onto the diseased cells. In these therapies, targeted cells are affected by oxidative damage that is initiated by illumination of the photosensitizing agent. However, a problem exists in that the drug photosensitizing agent may distribute to all tissues and if the application of light is not controlled, areas where the therapy is not desired may be negatively affected.